246 



Irrigation and Drainage 



nearly uniform head against either a weir or an opening as has 

 yet been devised. Its essential feature is a long, sharp lip, 

 over which the water may spill back into the canal in a thin 

 sheet, and thus maintain a nearly constant pressure back of 



Fig. 45. Spill-back method of dividing water. 



the lip of a weir or above an opening. But this arrangement 

 does not and cannot maintain a constant pressure where there 

 is any considerable fluctuation in the volume of water in the 

 main canal; and, since the depth of water above the opening 

 or lip of the weir must always be small, even a slight change 



